There's humor to be found in every culture. People do it because they see practices or commonalities among a culture that is foreign to them. One way to talk about these differences is to make fun of them. Americans are fat, lazy, ignorant, weak, materialistic, etc. There are plenty of successful, talented, very smart Indians. Indian culture, specifically their health and religion is much more bound to reality than anything Americans have to offer. On the other hand, some Indians do shit in the street, commonly do garbage tech jobs, are easily aggravated, and have a funny accent. I'm sure every person, American, Asian, African, etc. mocks accents in their native language. Usually it's not negative, more like playing a character for humor. People impersonate celebrities as a joke, most of the time it's because they like the person, or it just sounds funny. No ill intent.
Regarding cultural differences, it's easier to focus on the negatives for an outsider. The best way to disarm someone is to go along with the joke. It shows them you're not offended by what they say, even if you secretly are. Usually they'll drop it or see that you're cool and can take a joke. Don't highly value the opinions of retarded Americans. The only reason Americans feel above Indians is because they're in a cushy position because everything was built for them. Had they been dealt a different hand of cards, they wouldn't feel superior. Don't change yourself for people, everybody is focused on themselves with little regard of others. That mindset includes the people you want to change yourself for.